Apparatus for developing latent electrostatic images

ABSTRACT

Powder cloud development apparatus for developing latent electrostatic images on an insulating member, such as a photoconductive layer, wherein a cloud of charged developer particles is introduced into the development chamber through a first port in the chamber wall and air is introduced through a second port in the chamber wall opposite said first port. A baffle having a conductive upper portion is above the ports and below the insulating member, the baffle extending between the opposed walls. A bias potential is applied to a conductive substrate overlying said insulating member, the baffle allowed to electrically float whereby a substantially uniform electrostatic field is maintained between the insulating member, the conductive baffle portion, and the chamber bottom. Means are provided for automatically cleaning the baffle and chamber bottom, the developer particles removed during the cleaning cycle being collected in a sump. Spatter shields are provided to minimize toner spatter on the insulating member.

J eromin Dec. 9, 1975 APPARATUS FOR DEVELOPING LATENT ELECTROSTATIC IMAGES [75] Inventor: Lothar S. Jeromin, Sierra Madre,

Calif.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: Jan. 15, 1973 21 Appl. No.: 323,666

[44] Published under the Trial Voluntary Protest Program on January 28, 1975 as document no.

[52] US. Cl. 118/637; 118/629; 427/21 [51] Int. Cl. G03G 13/08 [58] Field of Search 117/175; l18/DIG. 5, 627, 118/629, 637

Primary Examiner-Mervin Stein Assistant ExaminerLeo Millstein Attorney, Agent, or FirmJames J. Ralabate; Terry J. Anderson; Irving Keschner [57] ABSTRACT Powder cloud development apparatus for developing latent electrostatic images on an insulating member, such as a photoconductive layer, wherein a cloud of charged developer particles is introduced into the development chamber through a first port in the chamber wall and air is introduced through a second port in the chamber wall opposite said first port. A baffle having a conductive upper portion is above the ports and below the insulating member, the baffle extending between the opposed walls. A bias potential is applied to a conductive substrate overlying said insulating member, the baffle allowed to electrically float whereby a substantially uniform electrostatic field is maintained between the insulating member, the conductive baffle portion, and the chamber bottom. Means are provided for automatically cleaning the baffle and chamber bottom, the developer particles removed during the cleaning cycle being collected in a sump. Spatter shields are provided to minimize toner spatter on the insulating member.

19 Claims, 4 Drawing Figures Sheet 1 of 4 US. Patent Dec. 9, 1975 US. Patent Dec. 9, 1975 Sheet 2 of4 3,924,568

US. Patent Dec. 9, 1975 Sheet4 of4 3,924,568

FIG. 4

APPARATUS FOR DEVELOPING LATENT ELECTROSTATIC IMAGES BACKGROUND or THE INVENTION Xeroradiography, as disclosed in. U.S. Pat. No. 2,666,144, is a process wherein an object is internally examined by subjecting the object to penetrating radiation. A uniform electrostatic charge is deposited on the surface of a xerographic plate and a latent electrostatic image is created by projecting the penetrating radiation, such as X-ray or gamma rays, through the object and onto the plate surface. The latent electrostatic image may be made visible by contacting the latent electrostatic image on the plate surface with fine powdered particles electrically charged opposite to the latent electrostatic image pattern on the plate in order to develop a positive image. The visible image may be viewed, photographed or transferred to another surface where it may be permanently affixed or otherwise utilized. The entire processing is dry, and no dark room is necessary.

Xeroradiography in recent years has been utilized to examine the extremities, the head, and to detect breast cancer in women. In examination of breasts wherein soft tissue comprises most of the breast area, xeroradiography, or xeromammography as it is generally called, provides greater resolving power than the conventional roentgenographic film and greater image detail is achieved. A wide range of contrast is seen on the xeroradiographic plate as compared to the conventional roentgenographic films so that all the structures of the breast from the skin to the chest wall and ribs may be readily visualized. Besides providing better contrast, xeromammography detects small structures like tumor calcification and magnifies them more than conventional film, is quicker, less expensive, gives greater detail and requires less radiation than prior nonphotoconductive X-ray techniques.

The technique of powder cloud development, as disclosed in U.S. Pat. No. 2,711,481, has been utilized to develop xeroradiography plates. This development technique'is preferred in xeromammography because discontinuities in the object being examined are readily developed. The charged surface of the plate is disposed facing a chamber area in which a cloud of powder particles are introduced. The particles must be charged opposite to the polarity of the charge on the plate so that the particles may deposit upon the surface of the plate in an image configuration due to the action of the electrostatic forces of the latent electrostatic image on the plate acting on the charged particles in the powder cloud. Various prior art techniques for charging the powder cloud include turbulently flowing the powder particles in air through a nozzle, tube or the like to triboelectrically charge the particles or by passing the particles through a corona discharge area comprising a fine needle or fine wire and a grounded exectrode as disclosed in U.S. Pat. No. 2,725,304.

U.S. Pat. No. 3,640,246 describes a powder cloud apparatus for developing latent electrostatic. images wherein an ion cloud and powder cloud are introduced into the development chamber through opposite walls and meet under a baffle, extending between the opposed walls, whereby the clouds are thoroughly mixed. A grid electrode, positioned between the plate carrying the latent electrostatic image and the baffle, is included, the grid being utilized to control image contrast and quality. The grid, by appropriate biasing during the development cycle, separates particles charged to an undesired polarity and accelerates the particles of the desired polarity to the surface of the photoconductor.

The development chamber described in the aforementioned patent, although satisfactory in most respects, has certain deficiencies associated therewith. For example, it has been determined that the ion generators and grid electrode do not contribute significantly to image development. In addition, after extended use the baffle becomes contaminated with toner, contributing to poor image quality by effecting a nonuniform electrostatic field distribution within the chamber. The filter bags, utilized to remove excess toner after the development cycle, clog rapidly, causing a machine dirt problem and increasing the necessity of maintenance calls. Finally, toner spatter at times degraded the image quality to a point that misinterpretation during image diagnosis would occur.

SUMMARY OF THE INVENTION The present invention provides novel apparatus for developing a latent electrostatic image formed on the surface of an insulating member. In particular, a cloud of charged developer particles is introduced into the development chamber through a first port in the chamber wall and gas is introduced through a second port in the chamber wall directly opposite said first port. A baffle having a conductive upper portion is positioned above the ports and below the surface of the insulator, the baffle extending between the opposed walls. A bias potential is applied to a conductive substrate overlying the insulating member, the baffle being allowed to electrically float whereby a substantially uniform electrostatic field is maintained between the insulating member and the conductive baffle portion and the chamber bottom. Means are provided for automatically cleaning the baffle and chamber bottom, the developer particles removed during the cleaning cycle being collected in a sump. Spatter shields are provided to minimize toner spatter on the insulating member.

It is an object of the present invention to provide improved apparatus for developing latent electrostatic images formed on the surface of an insulating member.

It is a further object of the present invention to provide improved powder cloud apparatus for developing latent electrostatic images formed on the surface of an insulating member wherein a cloud of charged toner particles are mixed with a gas under a baffle, the baffle having a conductive portion. A. bias potential is applied to a conductive substrate overlying the insulating member while the baffle conductive portion is electrically floating whereby a substantially uniform field is maintained between the baffle and the surface of the insulating member and the chamber bottom.

It is still a further object of the present invention to provide an improved powder cloud development chamber for developing latent electrostatic images formed on the surface of an insulating member wherein a cloud of charged toner particles are mixed with gas under a baffle having aconductive portion. A bias potential is applied to a conductive substrate overlying the insulating member while the baffle conductive portion is electrically floating whereby a substantially uniform field is maintained between the baffle and the surface of the insulating member and the chamber bottom. Means are provided for automatically cleaning the baffle and chamber bottom, the toner particles removed during the cleaning cycle being collected in a sump, for minimizing image degradation due to toner spatter and the effects of edge fields.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention as well as further objects and features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a simplified cross-sectional view of a powder cloud development apparatus incorporating the teachings of the present invention;

FIG. 2 is an isometric view, in partial cross-section, of the development chamber of the present invention;

FIG. 3 is a cross-sectional view of the development chamber along line 33 of FIG. 2 with the wiper blades in position for cleaning the chamber bottom; and

FIG. 4 is an exploded isometric view of the toner sump and filter housing components with the apparatus shown in FIGS. 1-3 mounted thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1, 2, 3 and 4, development means 100 includes holding means 102 positioned to receive a xerographic plate 104 having a conductive backing member 106 and downwardly facing photoconductive layer 108 therein. The development means of FIGS. 1-4 herein may be utilized in the automated flat plate xerographic processing system described in U.S. Pat. No. 3,650,620. As more fully described in the U.S. Pat. No. 3,650,620, means are provided to advance a latent electrostatic image-bearing xerographic plate into the development means and to advance the xerographic plate out of the development means after the latent electrostatic image has been converted to a corresponding xerographic powder image. Portions of the U.S. Pat. No. 3,650,620 which are necessary for complete understanding of the present invention or to provide sufficient disclosure to understand the more fully automated operation of the development chamber described herein are incorporated by reference.

The walls 114 of development chamber 104 tenninate, about the upper periphery thereof, in an outwardly extending lip 116. Attached to lip 116 is lip 115 and a gasket supporting member 118 having a gasket (or seal) 120 on the upper portion thereof. Gasket supporting member 118 is electrically biased to the same potential as conductive backing member 106 to reduce the effect of edge fields via potential source 121. Strong edge fields would degrade image quality along the edges of photoconductive layer 108. A xerographic plate 104 is positioned by appropriate transport mechanisms adjacent the development station, the xerographic plate being lowered by elevator means (not shown) such that gasket 120 is caused to seat against the non-photoconductive portions 103 of the conductive backing member 106 whereby a toner tight development chamber is defined.

Below the xerographic plate and mounted on a support bracket (not shown) is a canopy shaped baffle 130 having an upper portion 132 made of conductive material and a lower portion 134 made of insulator material. Pressurized gas, such as air, from source 133, is introduced via conduit 135 and manifold 136 into the development chamber through six air intake holes, or ports, 137 located in one wall of the development chamber,

the gas exiting to that portion of the development chamber beneath baffle 130. Extending through the side wall of the development chamber directly opposite ports 137 is a toner entrance port 138, also positioned beneath baffle 130. Port 138 is connected to powder cloud generator 139, shown in simplified form. A powder cloud generator which may be utilized in the present invention is disclosed in U.S. Pat. No. 3,648,90l.

A purge duct 140 is located in the bottom wall of the development chamber through which unused toner is withdrawn during the purge cycle. Duct 140 is mounted to duct support means 141 which in turn is in communication with filter means 142 and blower means 144. At the beginning of the purge cycle, the blower is in communication with the development chamber through toner filter means 142 and duct 140 whereby unused air borne toner is withdrawn from the development chamber. After the purge cycle, the purging means is removed from communication with the development chamber.

A bias potential is applied to the conductive backing member 106 and gasket supporting member 118 by bias supply 121, the conductive upper portion 132 of baffle 130 assuming a potential between ground and the bias potential applied to conductive backing member 106 (i.e., portion 132 assumes the potential gradient line through the baffle 130) whereby a substantially uniform electrostatic field is maintained between the photoconductive layer 108 and the upper portion 132 of the baffle 130 and between photoconductive layer 108 and the portions of the chamber bottom visible to the layer (the'portions of the chamber bottom directly below baffle 130 being electrically shielded from layer 108), allowing substantially identical image quality over an extended period of operation. The electric field established by the potential applied to conductive backing member 106 establishes field lines normal to the photoconductor layer 108 whereby the phenomenon of edge deletion can be controlled and also acts to accelerate the movement toward layer 108 of those particles of opposite polarity to the bias potential which are between the baffle and photoconductor layer 108.

Development means is of the powder cloud type wherein a fine cloud of charged toner particles is created by a powder cloud generator (not shown) as disclosed in U.S. Pat. Nos. 2,8l2,833 or 2,862,646 and blown into the development chamber 104 through port 138. The powder cloud so generated by the powder cloud generator is then mixed with the pressurized gas through ports 137. The powder cloud and gas meet under baffle and are thoroughly mixed. Because of the flow rates of the powder cloud generator and the gas, the charged powder cloud within the development chamber is caused to swirl out from under baffle 130 toward the upper portion of the development chamber whereby the charged toner particles are attracted to the latent electrostatic image of the photoconductive layer 108 whereby the latent image is developed. In the preferred embodiment, the toner cloud generator is pulsed a plurality of times to fill the development chamber with a charge of toner particles. The air flow is preferably maintained at a constant rate or, alternately, may be pulsed simultaneously with the toner (powder) cloud generator.

At the end of the development cycle, i.e., after the toner is introduced into the development chamber, and during which the latent electrostatic image has been made visible by the attraction of oppositely charged toner particles, the development chamber automatically is in communication with blower 144 via filter means 142. Air is drawn into the chamber to entrain unused toner via purge intake filter 150 and the airborne toner is collected in filter bag 142. At the end of the purge cycle, the xerographic plate is removed from the development chamber to an image transfer station whereat the powder image on the plate is transferred to receiving material, such aspaper.

The baffle 130, which defines the zone in which the toner cloud and the gas flow initially mix, is dimensioned and shaped to provide a uniform distribution of toner in the upper section of the development chamber as each cloud is moved out from under:it during succeeding pulses of the toner and/or gas supply apparatus. In the preferred embodiment, the baffle extends completely between the opposed sidewalls. In general, this movement of the charge powder cloud is affected by the kinetic energy imposed on the toner particles from the powder cloud generator system and by the conduction currents set up by the continuous flow of air. The shape of the baffle should be such as to provide for proper charged powder cloud movement to the development zone adjacent the charged xerographic plate.

The bias polarity applied to conductive backing member 106 enables the development chamber to develop positive and negative images. The polarity distribution of the toner blown into the chamber by the powder cloud generator is about 50:50 (positiveznegative). When a positive image is selected, a positive bias, typically +1,95O volts, is applied to backing member 106. The electrostatic field is such that the negatively charged toner is attracted to the image area on photoconductive layer 108 (carrying position charge) and background area (discharged area). Since the field strength is greater above the image area than above the background area, more toner is collected in the image area. The positively charged toner settles on the baffle 130 and the chamber bottom. When a negative image is selected, a.negative bias, typically 3,150 volts, is applied to the backing member 106. The negative backing member bias is greater than the greatest positive charge potential on the surface of layer 108. Therefore, the net potential on the backing member bias and the charge combined is always negative for any area on the layer 108. The positive toner particles are therefore now attracted electrostatically to the xerographic plate, the negative toner particles accumulating on the baffle '130 and chamber bottom. Since the field strength in the discharged or background area is now higher than in the image area, the image density is greater in the background which corresponds to a negative image.

The bias potential applied to gasket support member 118 via lead 117 reduces the effect of edge fields which may degrade image quality along the edges of the photoconductive layer 108. An insulating material 113 (FIG. 3) is included between lips 115 and 116 to insulate the chamber walls from the bias applied to gasket support member 118. I

The use of an electrically floating baffle portion 132 allows the potential of portion 132 to assume a value between ground and the potential on conductive backing member 106 thereby providing a substantially uniform field between baffle portion 132 and photoconductive layer 108 (and between layer 108 and the visible chamber bottom area) while eliminating the necessity of a separate grid element (and associated power supply) as described in US. Pat. No. 3,640,246 with the attendant cleaning and maintenance problems associated with its use. 7

Although filter bag 142 removes substantially most of the unused airborne toner remaining after the image is developed, the unremoved toner accumulates on the baffle and the bottom surface of the development chamber. Prolonged deposition of toner particles on the baffle and chamber bottom has a deletorious effect on the operational characteristics of the development chamber. Therefore, in accordance with the present invention, mechanical cleaning of the baffle portion 132 and the chamber bottom is provided, the cleaning being done automatically and in sequence.

, The cleaning of the accumulated toner at the bottom of the, development chamber and on the baffle minimizes the effect of the electrostatic field changes in the development chamber caused by the charged toner particles at the bottom of the development chamber and on the baffle portion 132.

In order to provide automatic and sequential cleaning of baffle portion 132 andthe chamber bottom, a dual wiper blade system is provided. This system comprises a parallel pair of toothed, or notched, belts 154 and 156, driven by a pair of timing pulleys 158 and 160, respectively (the second timing pulley associated with each belt is not shown) and wiper blades 162 and 164 mounted to belts 154 and 156v as shown. The position and size of the drive belt system is such that after the purge cycle, the wiper blades 162 and 164 automatically clean in sequence, the baffle upper portion 132 and the chamber bottom after the drive shafts 169 and 171, driving the corresponding timing pulleys, are automatically caused .to rotate. For example, assuming the wiper blade assembly travels in the direction of arrow 170, blade 164 will initially clean the surface, of baffle portion 132, traverse the circumferenceof timing pulley 158 and start to travelin' the direction of a rrow 172, whereby wiper blade 162 cleans the chamber bottom. (FIG. 3 shows the wiper assembly in position for cleaning the chamber bottom.) 7

It has been found advantageous to add an additional toner collecting means to collect the wiped toner, thereby increasing the overall toner collecting capacity of the apparatus. In order to accomplish this, a toner sump 152 is'included with the development apparatus of the present invention. The toner sump is utilized to collect toner wiped from baffle portion 132 and the bottom of the chamber after the purge cycle is completed and before the next development cycle is initiated.

Spatter shields and 182 are provided to minimize the effect of toner accumulated in the far corners of the chamber from being broken loose and projected by the discharged toner blown onto the photoconductive layer 108 as successive charges of toner are introduced into the development chamber. Guide means 188, 190, 192 and 194 are provided to guide the wiper blades adjacent thereto as they traverse the baffle and chamber bottom and also act to apply pressure on the blades whereby a contact force between the wiper blades and the surfaces to be cleaned is maintained.

In operation, when switches associated with the development chamber indicate that the xerographic plate is in a toner tight relationship with the development chamber, the development cycle is initiated by activating master timing means (not shown). Initially, the bias potential 121 is applied to conductive backing member 7 106 and gasket support member 118, pressurized air is continuously supplied to ports 137 and the toner powder cloud generator is pulsed one or more times to fill the development chamber with a charge of toner particles; the air and toner particles mixing under baffle 130 and develop the latent electrostatic image as explained hereinabove. At the end of the development cycle, the master timing means causes the development chamber to be in communication with the blower 144 and filter 142; air drawn through filter 150 entraining unused airborne toner. In this manner, unused toner is purged from the development chamber. After purge is complete, the xerographic plate is removed from the chamber by elevator means (not shown). By well known xerographic principles, the powder image on the plate is subsequently transferred to a support sheet, the powder image is then permanently fused to the sheet, the plate being then cleaned and processed for subsequent reuse.

After the xerographic plate is moved from the development chamber, the master timing means initiates the wiping and purge sequence whereby wiper blade 162 cleans the surface of baffle portion 132 and blade 164 cleans the chamber bottom, the wiped toner falling into toner sump 152, the stirred airborne toner collected in filter bag 142.

It should be noted that the present invention is not limited to medical diagnostic applications but may be utilized in industrial applications (i.e., non-destructive testing of an article of manufacture using X-ray imaging) and may be utilized in any system which requires the development of a latent electrostatic image formed on an insulating surface.

While the invention has been described with reference to its preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the present invention without departing from its essential teachings.

What is claimed is:

1. Apparatus for developing a latent electrostatic image formed on the surface of an insulating member comprising:

a chamber having a pair of opposed sidewalls,

means for supporting said insulating member, said surface facing said chamber,

means for supplying a cloud of charged developer particles to said chamber through a port in one of said opposed sidewalls,

means for supplying gas through a port in the other of said opposed sidewalls,

a baffle overlying said ports through which said developer particles and said gas are introduced into said chamber whereby said gas and developer particles mix under said baffle, said baffle comprising a conductive upper portion which is electrically floating and which faces the surface of said insulating member, said insulating member overlying a conductive substrate, and

means for biasing said conductive substrate to a predetermined potential whereby the electric field between said surface and said conductive baffle portion and said chamber bottom is substantially uniform.

2 Apparatus as defined in claim 1 wherein said insulating member comprises a photoconductive layer overlying said conductive substrate, the latent electrostatic image being formed on the surface of said photoconductive layer.

3. Apparatus as defined in claim 1 further including means for cleaning residual developer particles from the conductive upper portion of said baffle.

4. Apparatus as defined in claim 3 wherein said cleaning means sequentially cleans residual developer particles from the bottom of said chamber.

5. Apparatus as defined in claim 4 further including means for collecting developer particles cleaned from said baffle and said chamber bottom.

6. Apparatus as defined in claim 1 further including means for reducing developer particle spatter on the surface of said insulating member.

7. Apparatus as defined in claim 1 wherein said baffle extends between said opposed sidewalls.

8. Apparatus as defined in claim 1 wherein said biasing means applies a positive or negative potential to said conductive substrate whereby a negative or positive image, respectively, is developed.

9. Apparatus for developing a latent electrostatic image formed on the surface of an insulating member comprising:

a chamber having a pair of opposed sidewalls,

means for supporting said insulating member, said surface facing said chamber,

means for supplying a cloud of charged developer particles to said chamber through a port in one of said opposed sidewalls,

means for supplying gas through a port in the other of said opposed sidewalls,

a baffle overlying said ports through which said developer particles and said gas are introduced into said chamber whereby said gas and developer particles mix under said baffle, said baffle comprising a conductive upper portion which is electrically floating and which faces the surface of said insulating member, said insulating member overlying a conductive substrate, and

means for biasing said conductive substrate to a predetermined potential whereby the electric field between said surface and said conductive baffle portion and said chamber bottom is substantially uniform, said biasing means also being applied to said support means whereby the effects of edge fields on the developed image are minimized.

10. Apparatus for developing a latent electrostatic image formed on the surface of a plate member, said plate member comprising an insulating member overlying a conductive substrate, comprising:

a chamber having a pair of opposed sidewalls and support means for supporting said insulating member such that said insulating surface faces said chamber, said support means including a conductive support member,

means for supplying a cloud of charged developer particles to said chamber through a port in one of said opposed sidewalls,

means for supplying gas through at least one port in the other of said opposed sidewalls,

a baffle overlying said ports through which said developer particles and said gas are introduced into said chamber whereby said gas and developer particles mix under said baffle and are caused to move towards said insulating surface to develop said latent electrostatic image, said baffle comprising a conductive upper portion, said conductive upper 9 portion electrically floating and facing the surface of said insulating member, and

means for simultaneously biasing said conductive substrate and said conductive support member whereby the electric field between said insulating member and said baffle and said chamber bottom is substantially uniform and the effects of edge fields are minimized.

11. Apparatus as defined in claim 10 further including means for cleaning residual developer particles from the conductive upper portion of said baffle after said plate member is removed from said chamber.

12. Apparatus as defined in claim 11 wherein said cleaning means sequentially cleans residual developer particles from the bottom of said chamber.

13. Apparatus as defined in claim 12 wherein said baffle extends between said opposed sidewalls.

14. Apparatus as defined in claim 13 wherein said cleaning means comprises a cleaning member having first and second cleaning surfaces, said first and second cleaning surfaces extending between said opposed sidewalls and further including means for transporting said cleaning member across the surface of said conductive baffle portion in a first direction, said first cleaning sur- 10 face cleaning the surface of said conductive baffle portion.

15. Apparatus as defined in claim 14 wherein said cleaning member is transported in a second direction, opposite to said first direction, subsequent to the cleaning of the surface of said conductive baffle portion, said second cleaning surface thereby cleaning the bottom of said chamber.

16. Apparatus as defined in claim 15 further including means operatively associated with said chamber bottom for directing said residual developer particles cleaned from said conductive baffle portion and said chamber bottom to a receiving means and means for receiving said residual developer particles.

17. Apparatus as defined in claim 16 further including means for reducing developer particle spatter on the surface of said insulating member.

18. Apparatus as defined in claim 10 wherein said biasing means applies a positive or negative bias potential to said conductive substrate and said conductive support member whereby a negative or positive image, respectively, is developed.

19. Apparatus as defined in claim 10 wherein said baffle extends completely between said opposed sidewalls. 

1. Apparatus for developing a latent electrostatic image formed on the surface of an insulating member comprising: a chamber having a pair of opposed sidewalls, means for supporting said insulating member, said surface facing said chamber, means for supplying a cloud of charged developer particles to said chamber through a port in one of said opposed sidewalls, means for supplying gas through a port in the other of said opposed sidewalls, a baffle overlying said ports through which said developer particles and said gas are introduced into said chamber whereby said gas and developer particles mix under said baffle, said baffle comprising a conductive upper portion which is electrically floating and which faces the surface of said insulating member, said insulating member overlying a conductive substrate, and means for biasing said conductive substrate to a predetermined potential whereby the electric field between said surface and said conductive baffle portion and said chamber bottom is substantially uniform.
 2. Apparatus as defined in claim 1 wherein said insulating member comprises a photoconductive layer overlying said conductive substrate, the latent electrostatic image being formed on the surface of said photoconductive layer.
 3. Apparatus as defined in claim 1 further including means for cleaning residual developer particles from the conductive upper portion of said baffle.
 4. Apparatus as defined in claim 3 wherein said cleaning means sequentially cleans residual developer particles from the bottom of said chamber.
 5. Apparatus as defined in claim 4 further including means for collecting developer particles cleaned from said baffle and said chamber bottom.
 6. Apparatus as defined in claim 1 further including means for reducing developer particle spatter on the surface of said insulating member.
 7. Apparatus as defined in claim 1 wherein said baffle extends between said opposed sidewalls.
 8. Apparatus as defined in claim 1 wherein said biasing means applies a positive or negative potential to said conductive substrate whereby a negative or positive image, respectively, is developed.
 9. Apparatus for developing a latent electrostatic image formed on the surface of an insulating member comprising: a chamber having a pair of opposed sidewalls, means for supporting said insulating member, said surface facing said chamber, means for supplying a cloud of charged developer particles to said chamber through a port in one of said opposed sidewalls, means for supplying gas through a port in the other of said opposed sidewalls, a baffle overlying said ports through which said developer particles and said gas are introduced into said chamber whereby said gas and developer particles mix under said baffle, said baffle comprising a conductive upper portion which is electrically floating and which faces the surface of said insulating member, said insulating member overlying a conductive substrate, and means for biasing said conductive substrate to a predetermined potential whereby the electric field between said surface and said conductive baffle portion and said chamber bottom is substantially uniform, said biasing means also being applied to said support means whereby the effects of edge fiElds on the developed image are minimized.
 10. Apparatus for developing a latent electrostatic image formed on the surface of a plate member, said plate member comprising an insulating member overlying a conductive substrate, comprising: a chamber having a pair of opposed sidewalls and support means for supporting said insulating member such that said insulating surface faces said chamber, said support means including a conductive support member, means for supplying a cloud of charged developer particles to said chamber through a port in one of said opposed sidewalls, means for supplying gas through at least one port in the other of said opposed sidewalls, a baffle overlying said ports through which said developer particles and said gas are introduced into said chamber whereby said gas and developer particles mix under said baffle and are caused to move towards said insulating surface to develop said latent electrostatic image, said baffle comprising a conductive upper portion, said conductive upper portion electrically floating and facing the surface of said insulating member, and means for simultaneously biasing said conductive substrate and said conductive support member whereby the electric field between said insulating member and said baffle and said chamber bottom is substantially uniform and the effects of edge fields are minimized.
 11. Apparatus as defined in claim 10 further including means for cleaning residual developer particles from the conductive upper portion of said baffle after said plate member is removed from said chamber.
 12. Apparatus as defined in claim 11 wherein said cleaning means sequentially cleans residual developer particles from the bottom of said chamber.
 13. Apparatus as defined in claim 12 wherein said baffle extends between said opposed sidewalls.
 14. Apparatus as defined in claim 13 wherein said cleaning means comprises a cleaning member having first and second cleaning surfaces, said first and second cleaning surfaces extending between said opposed sidewalls and further including means for transporting said cleaning member across the surface of said conductive baffle portion in a first direction, said first cleaning surface cleaning the surface of said conductive baffle portion.
 15. Apparatus as defined in claim 14 wherein said cleaning member is transported in a second direction, opposite to said first direction, subsequent to the cleaning of the surface of said conductive baffle portion, said second cleaning surface thereby cleaning the bottom of said chamber.
 16. Apparatus as defined in claim 15 further including means operatively associated with said chamber bottom for directing said residual developer particles cleaned from said conductive baffle portion and said chamber bottom to a receiving means and means for receiving said residual developer particles.
 17. Apparatus as defined in claim 16 further including means for reducing developer particle spatter on the surface of said insulating member.
 18. Apparatus as defined in claim 10 wherein said biasing means applies a positive or negative bias potential to said conductive substrate and said conductive support member whereby a negative or positive image, respectively, is developed.
 19. Apparatus as defined in claim 10 wherein said baffle extends completely between said opposed sidewalls. 